Purpose: The role of the Nutrition and Metabolism Core is to provide intellectual resources regarding the nutritional and metabolic issues that are relevant to drug users who may or may not be HIV infected, as well as to train staff at the four collaborating sites to perform appropriate nutritional and metabolic determinations that would not otherwise be available. Core services will include assistance in protocol development, training in selection and performance of appropriate measurement of body composition, selection of appropriate micronutrient levels to measure, measurement of antiretroviral levels, discussion of appropriate measures of lipid levels, training in measures of dietary intake, physical activity, health related quality of life and clinical assessment of nutritional status.